Disk array controller with connection path formed on connection request queue basis

ABSTRACT

A disk array controller having a first interface unit to a host computer, a second interface unit to a plurality of disk drives, a cache memory unit for temporarily storing data to be transferred to and from the disk drives, and a selector unit provided between the first and second interface units and the cache memory unit, wherein a plurality of connection requests from the first and second interface units are queued to preferentially process a connection request for a vacant access port to the cache memory unit.

CROSS-REFERENCE TO RELATED APPLICATION

1. The present application relates to subject matter described inapplication Ser. No. ______ filed on ______ entitled “MULTI-PROCESSORTYPE STORAGE CONTROL APPARATUS FOR PERFORMING ACCESS CONTROL THROUGHSELECTOR”, by Kenji YAMAGAMI, Kazuhisa FUJIMOTO, Yasuo KUROSU and HisaoHONMA, and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION

2. The present invention relates to a controller for controlling a diskarray which divides data and stores the data in a plurality of diskdrives.

3. As compared to an I/O performance of a main storage of a computer, anI/O performance of a sub-system using a magnetic disk as a secondarystorage has a processing ability inferior by about three to four digits.Reducing this difference, i.e., improving the I/O performance of thesub-system has been tried in various ways.

4. As one method of improving the I/O performance of a sub-system, asub-system has been proposed which is constituted of a plurality of diskdrives and data is divisionally stored in the disk drives, i.e., aso-called disk array system is known.

5. For example, according to one conventional technique (hereinaftercalled a first conventional technique), as shown in FIG. 2, a disk arraysystem is constituted of: a plurality of channel I/F units 111 forexecuting data transfer between a host computer 101 and a disk arraycontroller 2; a plurality of disk I/F units 112 for executing datatransfer between disk drives 120 and the disk array controller 2; cachememory units 115 for temporarily storing data of the disk drives 120;and shared memory units 114 for storing control information on the datain the disk drives 120 and on the disk array controller 2, wherein thecache memory units 115 and shared memory units 114 can be accessed fromall of channel I/F units 111 and disk I/F units 112.

6. According to the first conventional technique, the channel I/F units111 and disk units I/F units 112 are connected to the shared memoryunits 114 in one-to-one correspondence, and the channel I/F units 111and disk units I/F units 112 are also connected to the cache memoryunits 114 in one-to-one correspondence.

7. According to another conventional technique (hereinafter called asecond conventional technique), as shown in FIG. 3, a disk array systemis constituted of: a plurality of channel I/F units 111 for executingdata transfer between a host computer 101 and a disk array controller 3;a plurality of disk I/F units 112 for executing data transfer betweendisk drives 120 and the disk array controller 3; cache memory units 115for temporarily storing data of the disk drives 120; and shared memoryunits 114 for storing control information on the data in the disk drives120 and on the disk array controller 3.

8. The channel I/F units 111 and disk I/F units 112 are connected to theshared memory units 114 via a shared bus 130, and to the cache memoryunits 115 via a shared bus 131.

9. Request for high performance of a disk array system has been dealtwith by using a large scale disk array controller and high speedcomponents, e.g., by an increase in the number of processors and in thecache capacity, use of high performance processors, expansion of aninternal bus width, improvement on a bus transfer ability and the like.

10. With the second conventional techniques, however, it is becomingdifficult for the transfer ability of an internal bus to follow a largescale system and performance improvement.

11. In order to achieve a high memory access performance by improvingthe internal bus performance, it is conceivable that one-to-onecorrespondence between processors and memories similar to the firstconventional technique is preferable.

12. With this method, the internal bus performance improvesproportionally to the number of access paths connected to the memories.

13. However, the number of access paths connected to shared memories andcache memories increases in proportion to an increase in the number ofprocessors used in the system.

14. In order to maximize the internal bus performance, it is necessaryto efficiently control the accesses between each processor and eachmemory.

SUMMARY OF THE INVENTION

15. It is an object of the present invention to solve theabove-described problem and provide a disk array controller capable ofefficiently using access paths between processors and memories andhaving a high memory access throughput, particularly a high cache memoryaccess throughput.

16. In order to achieve the above object of the invention, a disk arraycontroller is provided which comprises: one or more interface units to ahost computer; one or more interface units to a plurality of diskdrives; and one or more physically independent shared memory units forstoring control information on data in the disk drives and on the diskarray controller, wherein the interface units to the host computer andthe interface units to the disk drives can access the shared memoryunits via a selector, and access paths are connected between theselector and the interface units to the host computer and to the diskdrives and between the selector and the shared memory units, and whereinthe selector unit includes:

17. a unit for connecting a plurality of input ports (access paths) fromthe interface units to the host computer and to the disk drives to aplurality of output ports (access paths) to the shared memory units;

18. a unit for storing connection requests from input ports to outputports in an arrival order of the connection requests; and

19. an arbitor unit for arbitrating a plurality of connection requestsand assigning an output port to a connection request from an input port.

20. The arbitor unit assigns, if a top connection request among theconnection requests stored in the arrival order is a connection requestto a vacant output port, the output port to the connection request;checks a second connection request, if the top connection request amongthe connection requests stored in the arrival order is a connectionrequest to an occupied output port, and assigns, if the secondconnection request is a connection request to a vacant output port, theoutput port to the second connection request; checks a third connectionrequest, if the second connection request is a connection request to anoccupied output port, and thereafter repeats an arbitration (assignment)of an output port to a connection request at the most by several timesequal to the number of vacant output ports.

21. In this invention, the shared memory unit includes physicallyindependent and duplicated first and second shared memory units, and theselector accesses both of the first and second shared memory units atthe same time.

22. Also in this invention, the shared memory unit includes a cachememory unit and a shared memory unit both physically divided, the cachememory unit temporarily storing data of the disk drives, and the sharedmemory unit storing control information on the cache memory unit and thedisk array controller;

23. the selector unit includes first and second selectors bothphysically independent, the first selector connecting the cache memoryunit, and the second selector connecting the shared memory unit;

24. the disk array controller includes physically independent accesspaths between the interface units to the host computer and to the diskdrives and the cache memory unit or the shared memory unit; and

25. at least the first selector includes the arbitor unit.

26. Also in this invention, the shared memory unit includes physicallyindependent and duplicated shared memory units, the shared memory unitincludes physically independent and duplicated shared memory units, andat least the selector accesses both the duplicated shared memory unitsat the same time and is provided with the arbitor unit.

27. Also in this invention, when the interface units to the hostcomputer or to the disk drives access the shared memory unit or cachememory unit, an address and a command are sequentially transferred, andthen after an access path to the shared memory unit or cache memory unitis established, data is transferred.

28. According to the invention, the selector unit disposed between theinterface units to the host computer and to the disk drives and theshared memory units can efficiently distribute access requests from theinterface units to the shared memory unit. It is therefore possible toimprove throughput of data transfer of the disk array controller.

BRIEF DESCRIPTION OF THE DRAWINGS

29.FIG. 1 is a block diagram showing the structure of a disk arraycontroller of this invention.

30.FIG. 2 is a block diagram showing the structure of a conventionaldisk array controller.

31.FIG. 3 is a block diagram showing the structure of anotherconventional disk array controller.

32.FIG. 4 is a block diagram showing the structure of a selector unit ofthe disk array controller of the invention.

33.FIG. 5 is a flow chart illustrating the operation to be executed bythe selector unit.

34.FIG. 6 is a flow chart illustrating the operation to be executed byan arbitor of the selector unit.

35.FIG. 7 is a sequence diagram illustrating data write into a sharedmemory unit or a cache memory unit.

36.FIG. 8 is a sequence diagram illustrating data read from a sharedmemory unit or a cache memory unit.

37.FIG. 9 is a block diagram showing the structure of another disk arraycontroller of the invention.

38.FIG. 10 is a block diagram showing the structure of another diskarray controller of the invention.

39.FIG. 11 is a block diagram showing the structure of another diskarray controller of the invention.

40.FIG. 12 is a diagram showing the details of the selector unit of thedisk array controller of the invention.

41.FIG. 13 is a block diagram showing the structure of a channel I/Funit.

42.FIG. 14 is a block diagram showing the structure of the channel I/Funits shown in FIGS. 10 and 11 of the disk array controller of theinvention.

43.FIGS. 15A and 15B are diagrams illustrating the operation at Step 403when a request for a vacant output port is issued.

44.FIGS. 16A and 16B are diagrams illustrating the operation at Step 403when a request for an occupied output port is issued.

45.FIG. 17 is a diagram illustrating the operation at Step 405.

DETAILED DESCRIPTION OF THE EMBODIMENTS

46. Embodiments of the invention will be described in detailhereinunder.

47.FIG. 1 shows a first embodiment of the invention.

48. A disk array controller 1 is constituted of channel I/F units 111,disk I/F units 112, selector units 113, shared memory units 114, andaccess paths 135 and 136.

49. An access path is constituted of data lines and control lines.Control signals such as a connection request (REQ) and acknowledgement(ACK) are transferred over the control lines.

50. As shown in FIG. 13, the channel I/F unit 111 is constituted of oneI/F (host I/F) 51 to the host computer, one micro processor 50, and oneshared memory access controller (SM access controller) 52 including oneaccess path I/F 54 to the shared memory units 114.

51. For the data write, the host I/F 51 divides data supplied from thehost computer 101 into packets and sends them to the SM accesscontroller 52. The SM access controller sends a plurality of packetssupplied from the host I/F 51 to the shared memory unit 114 via theselector unit 113 by using one access path.

52. For the data read, the SM access controller 52 sends a plurality ofpackets supplied from the shared memory unit 114 to the host I/F 51. Thehost I/F 51 generates one set of data from a plurality of packetssupplied from the SM access controller 52 and sends it to the hostcomputer 101.

53. The micro processor 50 controls data transmission/reception at thehost I/F 51 and SM access controller 52.

54. The disk I/F unit 112 is basically the same as the channel I/F unit111 shown in FIG. 13, and is constituted of one I/F (drive I/F) to aplurality of disk drives 120, one micro processor, and one shared memoryaccess controller (SM access controller) including one access path I/Fto the shared memory units 114. In this structure, the host I/F 51 shownin FIG. 13 is replaced by the drive I/F. For the data read/write, aprocess at least similar to the process described for the channel I/Funit 111 is executed.

55. The numbers of devices described above are only illustrative and arenot limited thereto.

56. The shared memory unit 114 stores data to be written in the diskdrive 120 and management information such as management information ofthe data and system information.

57. The selector unit 113 is connected to four access paths 135 to twochannel I/F units 111 and two disk I/F units 112.

58. The selector unit 113 is also connected to two access paths to thetwo shared memory units 114.

59. One selector unit 113 and the two channel I/F units 111 and two diskI/F units 112 connected to the selector unit 113 constitute one groupwhich is called a selector group.

60. In this embodiment, the disk array controller 1 has two selectorgroups 150. The number above mentioned is only illustrative and is notlimited thereto.

61. The number of access paths between the channel and disk I/F unitsand the selector unit and the number of access paths between theselector unit and the shared memory units have the relation describedabove. Therefore, the selector unit 113 selects only two requestscorresponding to the number of access paths 136 to the shared memoryunits 114, from the requests issued from the channel and disk I/F units111 and 112 via the four access paths 135, and processes the selectedtwo requests.

62. The number of access paths connected between one selector unit 113and the shared memory units 114 is set smaller than the number of accesspaths connected between the channel and disk I/F units 111 and 112 andthe selector unit 113, and the number of selector units 113 is setsmaller than the total number of channel and disk I/F units 111 and 112,as described above. It is therefore possible to reduce the number ofaccess paths connected to the shared memory units 114.

63. With this setting, the problems of an LSI pin neck and a packageconnector neck of the shared memory unit can be solved.

64. More specifically, one access path is constituted of several tenssignal lines so that if signal lines are directly connected between theI/F units and shared memory units, the number of signal lines becomesenormously. Therefore, such connection is impossible by using one LSIpackage. This is called a LSI pin connection neck.

65. Similarly, the number of pins of an input/output connector betweenthe shared memory units and the channel and disk I/F units becomesenormously. It is therefore very difficult to increase the number ofpins of the connector to such an enormous number. This is called a LSIpin neck.

66. The invention can solve such problems.

67. Next, the internal structure of the selector unit 113 will bedescribed.

68.FIG. 4 shows the internal structure of the selector unit 113.

69. The selector unit 113 has: an I/F port unit 210 to the channel I/Funits 111 and disk I/F units 112; an I/F port unit 211 to the sharedmemory units 114; a selector 206 for the connection between the I/F portunits 210 and 211; error check units 201 for checking input/output dataat the I/F port units 210 and 211; buffers 202 for buffering addresses,commands and data supplied from the channel and disk I/F units 111 and112; an address/command (ADR/CMD) decoder 203 for decoding addresses andcommands supplied from the channel and disk I/F units 111 and 112; aqueue management unit 204 for managing decoded results in an arrivalorder, as connection requests to the I/F port unit 211; and an arbitorunit 205 for executing arbitration in accordance with the connectionrequests registered in the queue management unit 204 and determining aconnection privilege to the I/F port unit 211.

70. The LSI pin neck and package connector neck of the shared memoryunit can be solved, as described above, by setting the number of portsof the I/F port unit 210 smaller than the number of ports of the I/Fport unit 211.

71. In this embodiment, the number of ports of the I/F port unit 210 isset to “4” and the number of ports of the I/F port unit 211 is set to“2”.

72.FIG. 12 shows the detailed structures of the address/command(ADR/CMD) decoder 203, queue management unit 204 and arbitor unit 205.

73. The address/command (ADR/CMD) decoder 203 has four buffers 220corresponding in number to the number of ports of the I/F port unit 210to the channel and disk I/F units 111 and 112, and stores commands (CMD)and addresses (ADR) supplied from I/F ports 210-1 to 210-4.

74. Each address has a length of four bytes, and the first one byteindicates an output port number (port No.). Each command has a length offour bytes, and the first one byte indicates an access type (read: RD,write: WR, duplicate read: 2R, duplicate write: 2W). If the sharedmemory unit 114 is duplicated, duplicate read and duplicate write areexecuted in some cases. Such duplicate access uses two ports at the sametime. It is therefore necessary to acquire use privilege of two ports.

75. A port number decoder 221 derives a requested port number from anaddress. In this embodiment, a port 0 is assigned “00” and a port 1 isassigned “11”. A command decoder 222 derives an access type from acommand. In this embodiment, RD is assigned “00”, WR is assigned “01”,2R is assigned “10”, and 2W is assigned “11”. A required port decisionunit 223 outputs the port number itself if the access type is not aduplicate access, and outputs “01” if it is a duplicate access.

76. A queue management unit 204 registers the port numbers output fromthe address/command (ADR/CMD) decoder 203 in the arrival order in amanagement table 224, this operation being called queuing. The arbitorunit 205 picks up one port number from the top of the management table224 and stores it in a buffer 227. A comparison unit 228 compares anoccupied port number in a buffer 226 with the required port number inthe buffer 227.

77. If both the port numbers are different, the required port number isoutput to a selector 206 as selector switch signals SEL0 and SEL1, andan order control unit 225 of the queue management unit 204 is instructedto advance (shifts) the queue order by “1”. If the port numbers areequal, the order control unit 225 is instructed to exchange the queueorder. An arbitration method, an order exchange method, and a queueorder shift method will be described later at the arbitration flow shownin FIG. 6 by using specific examples.

78. The lengths of an address and a command, the locations of the portnumber and command type in an address and command, assignment of bits tothe port number and command type, described above, are only illustrativeand are not limited thereto. If the shared memory unit 114 is notduplicated, a duplicate access does not occur so that the commanddecoder 222 and required port decision unit 223 are not necessary. Inthis case, an output of the port number decoder 221 is directly input tothe queue management unit 204.

79. Next, processes to be executed by the selector unit 113 will bedescribed.

80.FIG. 5 is a flow chart illustrating the operation to be executed bythe selector unit 113 when an access is requested to one port of the I/Fport unit 210 from the channel and disk I/F units 111 and 112.

81. First, at Step 301 the process waits for an access request (REQ ON)to be issued from the SM access controller in the channel I/F unit 111or disk I/F unit 112.

82. When an access request is received, an address (ADR) and a command(CMD) are decoded at Step 302.

83. At Step 303, it is checked whether there is any error in the address(ADR) and command (CMD). If there is an error, an error process isexecuted at Step 315 to thereafter return to Step 301 and enter theaccess request stand-by state.

84. If there is no error, the decoded results are queued at Step 304 asa connection request to the I/F port (211-1, 211-2) to the shared memoryunits 114.

85. Arbitration is performed in accordance with the queue contents.

86. At Step 305 the process stands by until the requested port of theI/F port unit 211 to the shared memory unit 114 is acquired.

87. If acquired, the selector unit 206 is switched at Step 306 toconnect one requested port of the I/F port unit 210 to the acquired oneport of the I/F port unit 211.

88. Next, at Step 307, an access request (REQ ON) is issued to theshared memory (SM) unit 114 and an address (ADR) and a command (CMD) aretransferred.

89. At Step 308 the process stands by until an access acknowledgement(ACK ON) is returned from the shared memory unit 114.

90. When the access acknowledgement (ACK ON) is received, at Step 309the access acknowledgement (ACK ON) is returned to the SM accesscontroller of the channel I/F unit 111 or disk I/F unit 112.

91. At Step 310, in the case of data write, data supplied from the SMaccess controller is transmitted to the shared memory unit 114.

92. In the case of data read, data supplied from the shared memory unit114 is transmitted to the SM access controller.

93. In the data read/write, an error is checked at Step 311.

94. If an error is found, an error process is executed at Step 315 tothereafter return to Step 301 and enter the access request stand-bystate.

95. If there is no error, it is checked at Step 312 whether a STATUSindicating the contents of data processing is received, and data istransmitted until the STATUS is received.

96. If the STATUS is received, at Step 313 the shared memory unit isinstructed to withdraw the access acknowledgement (ACK OFF) tothereafter return to Step 301 and enter the access request stand-bystate.

97. Next, an arbitration method to be performed at Step 304 will bedescribed. FIG. 6 is a flow chart illustrating an arbitration operation.

98. At Step 401 it is checked whether there is a vacant port, and ifnot, the process stands by until a vacant port appears.

99. If there is a vacant port at Step 401, the top connection requestamong the connection requests stored in an arrival order in themanagement table 224 of the queue management unit 402 is checked at Step402. More specifically, as shown in FIG. 15A, the port number #0 of “00”in the management table 224 is output to the buffer 227. The comparisonunit 228 compares the port number “00” with the occupied port number“11” registered in the buffer 226.

100. If it is judged at Step 403 that the connection request is issuedto a vacant output port, then the output port is assigned to the requestat Step 404. Namely, as shown in the example of FIG. 15A, if therequired port number “00” is not the occupied port number “11”, theswitch signal SEL0 is output to connect the IF port 210-3 to the IF port211-1 (port number “00”). The path formation of this example in theselector unit 206 is shown in FIG. 15B.

101. If it is judged at Step 403 that the top connection request amongthe connection requests stored in the arrival order in the managementtable 224 of the queue management unit 204 is a request for an occupiedoutput port, the top queue request is shifted to the (number of vacantports+1)-th order at Step 406 and thereafter the flow returns to Step401. More specifically, as shown in an example of FIG. 16A, if therequired port number “00” is the occupied port number “00”, then asshown in FIG. 16B the port number #0 of “00” in the management table 224is registered in the port number #1 and the port number #1 of “01” isshifted to the port number #0 (the port number #0 of “00” is set to the(number of vacant ports (“1”)+1=2)-th order, and thereafter the flowreturns to Step 401.

102. If it is judged at Step 403 that the requested port is a vacantport and an output port is assigned at Step 404, the queue order isadvanced by “1” to thereafter return to Step 401. Namely, as shown inFIG. 17, the port number #0 of “00” in the management table 224 isdiscarded, the port number #1 of “11” is shifted to the port number #0,the port number #2 of “01” is shifted to the port number #1, the portnumber #3 of “11” is shifted to the port number #2, and a new requiredport number “11” is registered in the port number #3.

103. The above-described output port assignment is repeated severaltimes equal to the number of vacant ports. If there is no vacant port,the process stands by at Step 401 until a vacant port appears.

104. By effecting the above mentioned control only when the data to berecorded to the magnetic disk device 120 to which a high throughput isrequired is transmitted, it becomes possible to prevent a bad influencefrom affecting to a transmission of control information to which a shortaccess time is required.

105. With the above control, it becomes possible to efficiently assignthe I/F ports (211-1, 211-2) to the shared memory units and realize datatransfer of high throughput.

106. In another embodiment, as shown in FIG. 9, the shared memory unit114 may be duplicated by using physically independent shared memoryunits 114-1 and 114-2 to form a duplicated area 160. More specifically,the same data is written in each of the duplicate shared memory units114-1 and 114-2. The shared memory unit may be duplicated wholly orpartially.

107. In a disk array controller 4 in which accesses (a duplicate access)from the selector unit 113 to the duplicate shared memory units 114-1and 114-2 are generated at the same time, it is checked at Steps 402 and403 shown in FIG. 6 whether an access is a duplicate access. In the caseof a duplicate access, if required two ports are vacant, these ports areassigned, whereas if not, the control advances to Step 406.

108. In this manner, reliability of data stored in the shared memoryunits 114-1 and 114-2 can be improved.

109. It is also possible to efficiently assign the I/F ports 211-1 and211-2 to the shared memory units 114-1 and 114-2 when data to be writtenin the disk drives 120 is transferred.

110. The structure of the disk array controller shown in FIG. 1 ischanged to that shown in FIG. 10. Namely, the shared memory unit 114shown in FIG. 1 is physically divided into cache memory units 115 fortemporarily storing data to be written in the disk drives 120 and sharedmemory units 116 for storing control information on the cache memoryunits 115 and a disk array controller 5, and a selector unit (CMselector unit) 123 connected to the cache memory units 115 and aselector unit (SM selector unit) 124 connected to the shared memoryunits are made physically independent. The structures of the selectorunits 123 and 124 are the same as the selector unit 113.

111. Access paths 135 and 136 between the channel I/F units 111 and diskI/F units 112 and the cache memory units 115 and shared memory units 114are made physically independent, and at least the CM selector units 123connected to the cache memory units 115 execute arbitration in the samemanner as the process flow shown in FIG. 5. The reason why the SMselector units 124 do not execute arbitration is as follows. The controlinformation on the cache memory units 115 and disk array controller 5 isstored in the shared memory units and has a small data amount.Therefore, it takes only a short time to use ports and these ports soonbecomes vacant. As a result, even if arbitration is not executed, thereis no practical problem.

112. In another embodiment, as shown in FIG. 11, a shared memory unit116 and a cache memory unit 115 may be duplicated by using physicallyindependent shared memory units 116-1 and 116-2 and cache memory units115-1 and 115-2 to form duplicated areas 160. In this case, in a diskarray controller 5 in which accesses (a duplicate access) from at leastthe CM selector unit 123 connected to the cache memory units to theduplicate cache memory units 115-1 and 115-2 are generated at the sametime, it is checked at Steps 402 and 403 shown in FIG. 6 whether anaccess is a duplicate access. In the case of a duplicate access, ifrequired two ports are vacant, these ports are assigned, whereas if not,the control advances to Step 406. These operations are performed by theCM selector units 123 connected to the cache memory units.

113. In this manner, reliability of data stored in the cash memory units115-1 and 115-2 and shared memory units 116-1 and 116-2 can be improved.It is also possible to efficiently assign the I/F ports 211-1 and 211-2to the cache memory units 115-1 and 115-2 when data to be written in thedisk drives 120 is transferred.

114.FIGS. 7 and 8 are flow charts illustrating the details of theprocess flow shown in FIG. 5, and showing the data flow when the diskarray controllers having the structures shown in FIGS. 1, 9, 10 and 11operate.

115. For the data write as shown in FIG. 7, at Step 501 the SM or CMaccess controller 52 or 53 issues an access request (REQ) to theselector unit 113, 123 or 124, and then at Steps 502 and 503 an address(ADR) and a command (CMD) are transferred. In the following description,the selector unit 113, 123 or 124 is simply called a selector unit.

116. At Steps 504 and 505, the selector unit executes arbitration, andthe selector 206 is switched to assign a port to the shared memory unit114, 116 or cache memory unit 115.

117. At Step 506, the selector unit issues an access request (REQ) tothe shared memory unit or cache memory unit, and then at Steps 507 and508 an address (ADR) and a command (CMD) are transferred.

118. At Step 509 a memory module to be accessed is selected in theshared memory unit 114, 116 or cache memory unit 115, and thereafter atStep 510 an access acknowledgement (ACK ON) is returned via the selectorunit to the SM or CM access controller 52, 53.

119. Upon reception of ACK ON, the SM or CM access controller 52, 53sends data at Step 511.

120. Upon reception of all of the data, the shared memory unit 114, 116or cache memory unit 115 executes a post-process at Step 512, andreturns at Step 513 a STATUS to the SM or CM access controller 52, 53via the selector unit.

121. Upon reception of the status, at Step 514 the selector unitinstructs the shared memory unit 114, 116 or cache memory unit 115 towithdraw the access acknowledgement (ACK OFF).

122. Upon reception of the STATUS, at Step 515 the SM or CM accesscontroller 52, 53 instructs the selector unit to withdraw the accessacknowledgement (ACK OFF).

123. The data read process at Steps 601 to 610 is the same as the datawrite process at Steps 501 to 510 as shown in FIG. 8.

124. A data read pre-process is executed by the shared memory unit 114,116 or cache memory unit 115 at Step 611.

125. At Step 612 data is transferred to the SM or CM access controller52, 53 via the selector unit.

126. After data is transferred, the shared memory unit 114, 116 or cachememory unit 115 executes a post-process at Step 613. At Step 614 aSTATUS is returned to the SM or CM access controller 52, 53 via theselector unit.

127. Upon reception of the STATUS, at Step 615 the selector unitinstructs the shared memory unit 114, 116 or cache memory unit 115 towithdraw the access acknowledgement (ACK OFF).

128. Upon reception of the STATUS, at Step 616 the SM or CM accesscontroller 52,53 instructs the selector unit to withdraw the accessacknowledgement (ACK OFF).

129. As described above, when the channel I/F unit 111 or disk I/F unit112 accesses the shared memory unit 114, 116 or cache memory unit 115,an address and a command are sequentially transferred and after anaccess path to the shared memory unit 114, 116 or cache memory unit 115is established (step 510 or 610), data it transferred. It is thereforeunnecessary for the selector unit to buffer transfer data. Therefore,the buffer 202 is not required, the control at the selector unit can besimplified, and throughput of accesses to the memory can be improved.

130. In each of the embodiments described above, although disk drivesare connected, the invention is not limited only to disk drives butother drives for various types of disk media may also be used.

131. The invention is not limited only to the disclosed embodiments, butit includes various modifications which fall in the spirit and scope ofappended claims.

What is claimed is:
 1. A disk array controller comprising: a firstinterface unit to a host computer; a second interface unit to aplurality of disk drives; a cache memory unit for temporarily storingdata to be stored in the disk drives or data stored in the disk drives;and a selector unit provided between said first and second interfaceunits and said cache memory unit, wherein said selector unit comprises:a plurality of first ports to be connected to said first and secondinterface units; a plurality of second ports to be connected to saidcache memory unit; and means for preferentially processing a connectionrequest for a vacant port of the second ports, among a plurality ofconnection requests issued from said first and second interface units,if some of the second ports are occupied.
 2. A disk array controlleraccording to claim 1 , wherein said selector unit further comprisesmeans for connecting the vacant port to said first port to which theconnection request for the vacant port of said second ports was input.3. A disk array controller according to claim 1 , wherein said selectorunit further comprises: a queue for queuing a plurality of connectionrequests; and means for sequentially checking whether each of theplurality of queued connection requests is a connection request for avacant port, starting from a first queued connection request.
 4. A diskarray controller according to claim 3 , wherein said selector unitfurther comprises means for shifting an order of queued connectionrequests to the (number of vacant ports+1)-th order, if a checkedconnection request corresponds to an occupied port.
 5. A disk arraycontroller according to claim 3 , wherein said selector unit furthercomprises means for connecting, if a checked connection requestcorresponds to a vacant port, the vacant port to the first port to whichthe connection request was input.
 6. A disk array controller accordingto claim 1 , wherein said selector unit includes the first ports largerin number than the number of second ports.
 7. A disk array controlleraccording to claim 1 , wherein said cache memory includes a memory unithaving a duplicate structure.
 8. A disk array controller according toclaim 1 , wherein said selector unit includes a selector circuit unithaving a duplicate structure.
 9. A disk array controller comprising:first interface units to a host computer; second interface units to aplurality of disk drives; a cache memory unit for temporarily storingdata to be stored in the disk drives or data stored in the disk drives;and a selector unit provided between said first and second interfaceunits and said cache memory unit, wherein said selector unit comprises:a plurality of first ports to be connected to said first and secondinterface units; a plurality of second ports to be connected to saidcache memory unit; and means for connecting, if a first connectionrequest for one port A of the second ports is issued from any one of thefirst ports, thereafter a second connection request for the port A isissued from any one of the first ports, and thereafter a thirdconnection request for another port B different from the port A of thesecond ports is issued from any one of the first ports, the port A tothe port from which the first connection request was issued, and theport B to the port from which the third connection request was issued.10. A disk array controller comprising: first interface units to a hostcomputer; second interface units to a plurality of disk drives; a cachememory unit for temporarily storing data to be stored in the disk drivesor data stored in the disk drives; and a selector unit provided betweensaid first and second interface units and said cache memory unit,wherein said selector unit comprises: means for connecting a pluralityof input ports from said first and second interface units to a pluralityof output ports to said cache memory unit; means for storing connectionrequests from said input ports to said output ports in an arrival orderof the connection requests; and arbitor means for arbitrating aplurality of connection requests and assigning an output port to aconnection request from an input port, said arbitor means: assigns, if atop connection request among the connection requests stored in thearrival order is a connection request to a vacant output port, theoutput port to the connection request; checks a second connectionrequest, if the top connection request among the connection requestsstored in the arrival order is a connection request to an occupiedoutput port, and assigns, if the second connection request is aconnection request to a vacant output port, the output port to thesecond connection request; checks a third connection request, if thesecond connection request is a connection request to an occupied outputport, and thereafter repeats an assignment of an output port to aconnection request at the most by several times equal to the number ofvacant output ports.
 11. A disk array controller according to claim 10 ,wherein said cache memory unit includes physically independent andduplicated first and second cache memory units, and said selector unitincludes means for accessing to both of said first and second cachememory units at the same time.
 12. A disk array controller according toclaim 10 , wherein: said cache memory units includes a cache memory unitand a shared memory unit both physically divided, said cache memory unittemporarily storing data of the disk drives, and said shared memory unitstoring control information on said first cache memory unit and the diskarray controller; said selector unit includes first and second selectorsboth physically independent, said first selector connecting said cachememory unit, and said second selector connecting said shared memoryunit; the disk array controller includes physically independent accesspaths between said first and second interface units and said cachememory unit or said shared memory unit; and at least said first selectorincludes said arbitor means.
 13. A disk array controller according toclaim 12 , wherein: said first cache memory unit includes physicallyindependent and duplicated first and second cache memory units; saidshared memory unit includes physically independent and duplicated firstand second shared memory units; and said first selector includes meansfor accessing both said first and second cache memory units at the sametime.
 14. A disk array controller according to claim 10 , furthercomprising means for sequentially transferring, when said first andsecond interface units access said cache memory unit, an address and acommand, and then after an access path to said cache memory unit isestablished, transferring data.